Several different therapeutic approaches have produced significant recovery of motor function in experimental models of spinal cord injury (SCI). These treatments include strategies designed to enhance axon regeneration and strategies targeted towards remyelination, tissue sparing and training the spared circuits. Their potential usefulness at the bedside, however, is critically dependent on nerve-muscle connectivity that not only remains functional but also functions as efficiently as possible. Surprisingly, however, the synaptic responses of muscles to SCI have received little attention, and the neuromuscular junctions (NMJs) caudal to SCI are assumed to remain intact. Our preliminary morphological analyses, however, suggest that NMJs in hindlimb muscles of adult rats paralyzed by SCI may be extremely dysfunctional. Furthermore, these studies imply that adult NMJs may be extraordinarily diverse and specific in their sensitivity to paralysis. In this R21 application, we will use fluorescent transgenic mice, in vivo time-lapse imaging, and combined electrophysiological and morphological analyses to determine (1) if there are multiple subpopulations of NMJs that differ in pre- and postsynaptic sensitivity to the SCI-elicited paralysis, and (2) if physiologically significant loss of nerve-muscle connectivity accompanies morphological instability of NMJs distal to SCI. These results will provide critical data to justify a larger grant application to: explore the molecular mechanisms underlying the unexpected diversity of mature NMJs;comprehensively assess the contribution of NMJ loss to motor deficits associated with SCI;and attempt to promote motor recovery by stabilizing NMJs. The proposed work therefore has the potential to establish a strong foundation for developing novel treatments for spinal cord injured patients. PUBLIC HEALTH RELEVANCE: Several different therapeutic approaches have produced significant recovery of motor function in experimental models of spinal cord injury. These treatments include strategies designed to enhance axon regeneration and strategies targeted towards remyelination, tissue sparing and training the spared circuits. The project seeks to provide a novel basis for motor deficits and recovery following spinal cord injury by identifying nerve-muscle connections, termed neuromuscular junctions, as novel therapeutic targets.